Various types of grinding mills are typically employed to grind solid materials such as minerals, limestone, gypsum, phosphate rock, salt, coke, biomass and coal into small particles for use in a wide range of processes such as for combustion in furnaces and for chemical reactions in reactor systems. There are many types and configurations of grinding mills including ball mills, roller mills and bowl type vertical grinding mills. The ball mills typically include a horizontal rotating cylinder containing a charge of tumbling or cascading balls. The roller mills are sometimes referred to as pendulum mills which include a support shaft rotationally supported by a bearing housing. One end of the shaft is coupled to a drive unit for rotating the shaft. An opposing end of the shaft has a hub mounted thereto. A plurality of arms extend from the hub. Each of the arms pivotally supports a roller journal which has a roller rotatingly coupled to an end thereof. The rollers rollingly engage the grinding ring. During operation of the roller mill, centrifugal forces drive the crushing members against the grinding ring. The crushing members pulverize the solid material against the grinding ring as a result of contact with the grinding ring.
As shown in FIG. 1, bowl type vertical grinding mills 100 typically include a body portion 110 in which a bowl assembly 112 is mounted for rotation. The bowl assembly 112 includes a grinding table 112T having a wear insert (e.g., a wear resistant liner) 112W secured therein. The wear insert 112W defines a grinding surface 116 thereon. Typically, these grinding mills 100 include three grinding rollers 118 each mounted on a suitably supported journal 120. The grinding rollers 118 interact with the grinding table 112T to effect the grinding of material interposed therebetween. After being pulverized, the particles of material are thrown outwardly by centrifugal force whereby the particles of material are fed into a stream of air that is entering the mill 100. The stream of air with the particles of material entrained therein flows into a classifier in which coarse particles of material are rejected from the air stream. These rejected coarse material particles are then returned to the grinding table 112T for further pulverization, while the fine particles of material are carried through the bowl mill 100 in the air stream, and exit along with the air.
As best shown in FIG. 2, the wear insert 112W is secured to the grinding table 112T with a clamp ring 115 located at a radially outward most portion of the grinding table 112T. A dam ring 114 extends axially upward from the clamp ring 115. The dam ring 114 is configured to prevent particles from being prematurely thrown out of the pulverizing range of the rollers 118 by centrifugal force created by the rotation of the bowl assembly 112. Thus, the dam ring 114 has been considered an essential component to ensure proper operation of all bowl mills.
As shown in FIG. 1, a classifier 122 is positioned on an upper end of the body portion 110 of the bowl mill 100 within and occupying an interior area defined by a cone 124. The classifier 122 may be a static classifier, a dynamic classifier or a hybrid classifier including those described in U.S. Pat. No. 7,267,293, which issued Sep. 11, 2007. The classifier 122 operates to effect a further sorting of the particles of material that remain in the air stream. Namely, those particles of pulverized material which are of the desired particle size pass through the classifier 122 and, along with the air, are discharged from the bowl mill 100 through outlets (not shown). Those particles of material which in size are larger than desired, i.e., coarse particles, are returned to the bowl assembly 112, via the cone 124, whereupon they undergo additional pulverization.
The prior art bowl mills 100 are generally suitable for producing fine particles, but cannot produce coarse particles coarser than 40-50 percent passing 200 mesh. As used herein, the percent of material passing a sieve screen refers to the amount of material smaller than that particular sieve opening. However, there is a need to produce a larger amount coarser particles in certain applications. For example, certain types of sorbents (e.g., limestone for a circulating fluidized bed (CFB) boiler) and biomass fuels are required to be ground or pulverized to coarser particle sizes such as for example, particles having a fineness of 99 percent passing a 1 or 2 mm sieve opening. In addition, such sorbents and biomass fuels have limitations on the amount of fine particles that are included in the pulverized product. For example, fine particles can be limited to 25 percent passing 200 or 325 mesh. For the case of sorbent applications, the fine particles can be blown out of the CFB boiler before completely reacting with sulfur. For biomass applications, the fine material less than 200 mesh (75 μm) is an explosive dust and should be limited. Thus, there is a rather narrow range of acceptable particle sizes for certain sorbents and biomass fuels. The prior art bowl mills 100 are generally not capable of producing a large percentage of coarse particles and tend to overgrind the particles. Such overgrinding causes an undesired increase in power consumption, a decrease in throughput of the bowl mill 100, and thus increasing the amount of sorbent usage for the case of sorbent application.
Based on the foregoing, there is a need for an improved bowl mill that is configured to produce such coarse particles in the acceptable range while limiting the amount of fine particles.